1. Field of the Invention
The present invention relates to a cemented lens group which can be used in various optical systems.
2. Description of the Prior Art
Achromatic lens groups consisting of at least one positive lens element and one negative lens element which are positioned closely to each other with a slight space therebetween are well-known in the art and used in various optical systems. In such types of achromatic lens groups, each of a convex surface and a concave surface of the two lens elements which face each other with a slight space therebetween has a strong surface power since a strong power is given to each of the positive lens element and the negative lens element to correct axial chromatic aberration. Due to this lens design, if one of the two lens elements becomes eccentric with respect to the other lens element, strong coma aberration and/or flare occur, substantially deteriorating the optical performance of the achromatic lens group. To prevent this problem from occurring, the rim of the lens group and the lens holder thereof have to be made with a high degree of precision so that none of the lens elements become decentered. However, this increases the cost of production.
On the other hand, another type of achromatic lens group which consists of at least one positive lens element and one negative lens element which are cemented to each other is known in the art and used in various optical systems. In this cemented type of achromatic lens group, two lens elements are cemented to each other by an adhesive after the two lens elements are precisely aligned. This achieves an achromatic cemented lens group in which substantially no deterioration in optical performance occurs. However, in the case of a cemented lens group consisting of two lens elements of different glass materials having different thermal expansion coefficients, a variation in temperature causes a difference in outside diameter between the two lens elements. This difference generates a shearing stress exerted on each cementing surface. The shearing stress becomes great, especially when an appreciable variation in temperature occurs. In this case, there is a high possibility that the cemented lens group may become deformed, or in the worst scenario, the cementing surfaces may come off each other (separate) due to the shearing stress.
The difference in the outside diameter between the two lens elements due to a variation in temperature increases as the outside diameter of the cemented lens group increases. Accordingly, the aforementioned problems in cemented lens group become more serious if the cemented lens group has a large diameter. Fluorite and low-dispersion glass have excellent achromatic characteristics, and are often used as materials of high-performance achromatic lens. However, such particular materials have a high thermal expansion coefficient, more than double the thermal expansion coefficient of a typical optical glass material. Accordingly, if a lens element made of such a material having a high thermal expansion coefficient and a lens element made of a typical optical glass material are cemented to each other, the aforementioned problems in the cemented lens group thereof become much more serious.
The present invention has been devised in view of the aforementioned problems, and accordingly, the present invention provides a cemented lens group consisting of at least two lens elements which are cemented to each other by an adhesive, wherein there is substantially no possibility that the cemented lens group deforms or that the cementing surfaces come off each other by a shearing stress exerted on the cementing surfaces. The present invention is of great value especially when providing a large-diameter cemented lens group, the lens elements of which are made of different glass materials having different thermal expansion coefficients, e.g., a lens element of a typical glass material and a lens element of fluorite or a low-dispersion glass.
The present invention provides a cemented lens group wherein the thickness of the adhesive layer between the lens elements can be easily managed, wherein the optical performance deteriorates little, even if the adhesive layer is elastically deformed.
For example, a cemented lens group is provided, including two lens elements, cementing surfaces of the two lens elements being cemented to each other by an adhesive, the adhesive forming an adhesive layer between the cementing surfaces, wherein filler made of minute solid material is dispersed throughout the adhesive layer.
The filler can be made from resin granules, glass fiber granules, or cut glass fibers.
It is desirable for the following condition (1) to be satisfied:
|xcex94xcex1xc2x7D/d| less than 0.03xe2x80x83xe2x80x83(1); 
wherein xcex94 xcex1 designates a difference in linear expansion coefficient between the two lens elements cemented to each other, D designates the diameter of each of the lens elements, and d designates a thickness of the adhesive layer having the filler included therein.
It is desirable for the following condition (1xe2x80x2) to be satisfied:
|xcex94xcex1xc2x7D/d| less than 0.01xe2x80x83xe2x80x83(1xe2x80x2). 
It is desirable for the following condition (2) to be satisfied:
d/D less than 0.002xe2x80x83xe2x80x83(2). 
It is desirable for the following condition (3) to be satisfied:
d greater than 0.015 mmxe2x80x83xe2x80x83(3). 
It is desirable for the following condition (4) to be satisfied:
d less than 0.2 mmxe2x80x83xe2x80x83(4). 
It is desirable for the following condition (5) to be satisfied:
D greater than 80 mmxe2x80x83xe2x80x83(5). 
It is desirable for the following condition (6) to be satisfied:
|xcex94xcex1| greater than 0.0000015xe2x80x83xe2x80x83(6). 
In another embodiment, a cemented lens group is provided, including two lens elements, cementing surfaces of the two lens elements being cemented to each other by an adhesive, the adhesive forming an adhesive layer between the cementing surfaces, wherein a spacer which determines the thickness of the adhesive layer is provided around the peripheral portions of the cementing surfaces.
The spacer can be provided discontinuously around the peripheral portions of the cementing surfaces.
The spacer can be provided continuously around the peripheral portions of the cementing surfaces.
The spacer can include a sandwiched portion which is sandwiched in between the peripheral portions of the cementing surfaces, and a peripheral portion which is provided around an edge portion of one of the two lens elements.
The spacer can be made from a resin film, a resin sheet, or from tin foil.
It is desirable for the following condition (1) to be satisfied:
|xcex94xcex1xc2x7D/d| less than 0.03xe2x80x83xe2x80x83(1); 
wherein xcex94 xcex1 designates a difference in linear expansion coefficient between the two lens elements cemented to each other, D designates the diameter of each of the lens elements, and d designates a thickness of the adhesive layer determined by the spacer.
It is desirable for the following condition (1xe2x80x2) to be satisfied:
|xcex94xcex1xc2x7D/d| less than 0.01xe2x80x83xe2x80x83(1xe2x80x2). 
It is desirable for the following condition (2) to be satisfied:
d/D less than 0.002xe2x80x83xe2x80x83(2). 
It is desirable for the following condition (3) to be satisfied:
d greater than 0.015 mmxe2x80x83xe2x80x83(3). 
It is desirable for the following condition (4) to be satisfied:
d less than 0.2 mmxe2x80x83xe2x80x83(4). 
It is desirable for the following condition (5) to be satisfied:
D greater than 80 mmxe2x80x83xe2x80x83(5). 
It is desirable for the following condition (6) to be satisfied:
|xcex94xcex1| greater than 0.0000015xe2x80x83xe2x80x83(6). 
It is desirable that the adhesive be a silicon resin made of an organic silicon compound and that the silicon resin be of an addition-reaction type. Alternatively, the adhesive is made of a silicon resin having elongation of at least 100 percent. The term xe2x80x9cElongationxe2x80x9d is a technical term of JIS (Japan Industrial Standard), and is represented by the following equation:
Elongation (%)=(Amount of Extension/Original Length)xc3x97100. 
Namely, for instance, elongation of 100% and 150% indicate a length which is two times of the original length, and a length which is two and half times of the original length, respectively.
The present disclosure relates to subject matter contained in Japanese Patent Application Nos. 2001-336164 and 2001-336166 (both filed on Nov. 1, 2001) which are expressly incorporated herein in their entireties.